The Structure of the Atom

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The Structure of the Atom

• History, Structure, Properties and Forces
• Chapter 4

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Atomic Structure
History, Structure and Properties of the Atom
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History of the Atom

• Great Thinkers (2,000yrs. Ago)
• Age of Reason and Thought
• Democritus vs. Aristotle view on the make-up of
  matter.

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Democritus(400b.c.)Greek Philosopher

• Observed matter to be made up of atoms.
• Atoms are the smallest form of matter.
• Atoms cannot be broken down.
• The types of atoms in matter determine its
  properties.

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Aristotle Greek Philosopher (300b.c.)

• Aristotle observed matter to be composed of
  one or more of these four elements
• air, earth, fire and water.
• Most people believed Artistole.
• Why?

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John Daltons Atomic Theory

• English school teacher
• Proved Democritus atoms hypothesis using the
  scientific method.
• His conclusions produced
• Daltons Atomic Theory

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Daltons Atomic Theory

• Atoms are the building blocks of all matter.
• 2. Atoms cannot be subdivided.
• 3. Each element has the same kind of atoms.
• 4. In a compound, the different atoms chemically
  combine in the same way (fixed composition).
• 5. Atoms cannot be created or destroyed just
  rearranged.

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Thomsons Cathode Ray Experiment

• J.J. Thomson (pg. 105)
• 1897 discovered electrons in gas atoms using a
  cathode ray tube.
• Determined electrons have a negative charge.
• Electrons have the same charge in all atoms.

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Thomsons Plum Pudding Model

• If atoms are made of electrons how come most
  matter does not shock us?
• Atoms must have positive particles, too.
• He proposed the Plum Pudding Atomic Model

An atom is equally made up of positive and
negative particles.
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Goldsteins Cathode Tube Experiment
chemed.chem.purdue.edu
reich-chemistry.wikispaces.com

• He discovered the positively charged particle,
  proton, using a cathode ray tube also.

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Rutherfords Gold Foil Experiment

• The laser beam consisted of positively charged
  alpha particles.
• Hypothesized that if the Plum Pudding model of
  the atom was correct then the
• charged alpha particles should deflect or bend
  slightly when in contact with the gold foil
  atoms.
• His experimental results revealed something
  different. Most alpha particles paths were not
  affected by any charge when in contact with gold
  foil. (Red straight lines on left diagram). Just
  a few deflected back at large angles. Knew it
  had to be of the same charge, because repelled.

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Rutherfords Conclusion

• Most of the atom is empty space.
• Small dense region composed of
• () charged particles.
• (Nucleus)

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Rutherfords Nuclear Atom Model

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Subatomic Particle Neutron

• What keeps the protons within the nucleus ?
• (Like particles repel each other)
• 1932 - James Chadwick discovers the nucleus also
  has neutral particles present. He called them
  neutrons.
• Neutrons have a significant mass like protons.

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Properties of Subatomic Particles
Atomic Subparticles Charge mass (g) mass (amu) atoms contribution
Electron (e-) () 9.11 x 10 -28 so tiny, no mass (10,000x smaller than p or no Contributes (-) charge
Proton (p) (-) 1.674 x10-24 1 atomic mass unit for each proton. Contributes () charge mass
Neutron (no) No charge 1.675 x10-24 1 atomic mass unit for each neutron Contributes mass minimizes repulsion force of p in nucleus
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Bell Ringer

• 1. Compare the different views Aristotle and
  Democritus had about what matter was made of.
• 2. Which Greek philosopher was correct ?
• 3. Which later scientist proved his view?

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Bell Ringer4. a. Identify and explain this
Atomic Model?b. Who concluded this atomic model?
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Bell Ringer

• 5. Look at the following experiment below.
• What was this experiment called?
• Who developed this experiment?
• Did his experiment prove or disprove the Plum
  Pudding Model?

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Modern Atomic Model
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Properties of Subatomic Particles
Atomic Subparticles Charge mass (g) mass (amu) atoms contribution
Electron (e-) () 9.11 x 10 -28 so tiny, no mass (10,000x smaller than p or no Contributes (-) charge
Proton (p) (-) 1.674 x10-24 1 atomic mass unit for each proton. Contributes () charge mass
Neutron (no) No charge 1.675 x10-24 1 atomic mass unit for each neutron Contributes mass minimizes repulsion force of p in nucleus
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Distinguishing Atoms

• atomic number The number of protons within an
  atoms nucleus.
• Atoms I.D.
• Atoms of the same element always have the same
  number of protons.
• Elements are arranged numerically on the periodic
  table based on their atomic number.

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Neutral Atoms

• Most matter in nature is neutral.
• (Doesnt shock us!)
• This means the atoms making up the matter is
  neutral. ?
• What are the two charged subatomic particles?
• p and e-
• For an atom to be neutral the of p of
  e-.

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Atoms Mass

• Mass number (amu)
• What contributes mass to the atom?
• Sum of protons and neutrons in nucleus of atom.
• Problem A Selenium(Se) has an atomic of 34
  and a mass of 80 amu. How many p, e-, and n0
  are there in selenium? p (proton) 34
  (atomic )
• e- (electron) 34
  (balance p)
• n0 (neutron) 80 34
  46

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Atoms Subatomic Particles

• Problem B A cesium (Cs) atom has a mass of 133
  and an atomic number of 55 amu. How many p, e-,
  and no are there?
• P 55 e- 55 no 78
• Problem C An iron atom has an atomic number of
  26 and 30 neutrons.
• How many p and e- are there?
• P 26 e- 26
• What is irons mass number?
• Mass number 56 amu

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An Elements Isotopes

• Isotopes atoms with different number of
  neutrons in their nucleus.
• Elements can have several isotopes.
• Ex. Carbon C-12 and C-14 (mass )
• Mass changes. (varied neutrons)
• The number of protons and electrons stay the
  same.

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An Elements Isotopes
C-12
C-14 of p, n0, e-? of p,
n0, e-?

• Do you think an elements isotopes have similar
  or different chemical properties?

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Elements Isotopes

• Oxygen has three isotopes. They are O-16,
• O-17, and O-18.
• What is the mass number for each isotope of
  oxygen in amu?
• 2. What is the atomic number for each isotope of
  oxygen?
• 3. What is the number of p, n0, and e- for each
  isotope of Oxygen ?

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Atomic MassElements average mass

• Atomic mass the weighted average mass of all the
  elements isotopes present within a naturally
  occuring sample of matter.
• Calculate Atomic Mass of an Element
• 1. Multiply the mass of each isotope by its
  natural abundance ( common occurance) in decimal
  form.
• 2. Add all the isotopes products together.

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Atomic Mass(Elements average mass)

• Carbon (C) has two isotopes
• Carbon-12 has a natural abundance 99
• Carbon-13 has a natural abundance 1
• Carbons atomic mass
• (12 x 0.99) ( 13x0.01) 11.88 0.13 12.01
  amu
• 
  (sig. figs.)
• 
  

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Atomic Mass

• What is the difference between an atoms mass
  number and an elements atomic mass?
• Calculate the atomic mass of the following
  magnesium isotopes.

Isotope Natural Abundance ()
I-127 80
I-126 17
I-128 3
Answer 126.86 126.9 (sig. figs)